This invention relates to a method of making improved, long wearing ignition distributor rotor electrodes. More particularly, the invention relates to a method of making rotor electrodes wherein specially adapted dielectric silica based particles are securely locked into the electrically conductive metal matrix of a rotor electrode body to suppress radio frequency interference.
Radio frequency interference (RFI) in vehicles powered by spark ignition internal combustion engines is caused, for the most part, by high voltage spark discharges across the rotor gap. The RFI source is a large, fast rise time impulsive current generated at the onset of the rotor gap breakdown. The higher the voltage required to break down the gap, the higher the intensity of the radio frequency interference noise. It is known that the breakdown voltage across a given rotor gap can be lowered by providing a source of initiatory electrons, e.g., a dielectric material, at the surface of the rotor electrode at the gap. It is theorized that these electrons are accelerated away from the electrode tip by the application of an electric field to collide with air molecules in the gap and ionize them. The presence of these ions increases the probability that the rotor gap will break down at a lowered voltage where RFI is substantially suppressed.
Certain prior art RFI suppressing distributor rotor electrodes have been made by mixing dielectric silica or glass particles with metal powder, compacting the powder mixture in the shape of the electrode, and sintering the compact to obtain a strong, dense electrode. A difficulty with such methods has been that the dielectric particles have necessarily been in the form of symmetrical small spherical powder particles or short fibers. They end up in the electrode in the same configuration and they are not strongly locked in the metal matrix. Although such electrodes suppress RFI noise generation, sparking at the rotor gap can dislodge the regularly shaped dielectric particles and cause excessive electrode wear and noise in the FM frequency band.
Accordingly, it is an object of my invention to provide a method of making a distributor rotor electrode wherein extremely small and irregularly shaped dielectric particles (basically silica) are formed in a conductive metal matrix. As a result, the dielectric particles are better locked in the metal matrix and reduce electrode wear and dielectric particle loss at the rotor gap.
It is a more specific object of my invention to provide a method of making a conductive metal distributor rotor electrode containing a distributed ultrafinely divided dielectric silica constituent. In such method the silica constituent is formed so that its shape is highly irregular and it is incorporated in the metal matrix so that it is tightly locked in. Copper is intimately associated with the silica, the copper being in its elemental or oxide form.